Chromium green-black hematite (basic chemical formula: Cr.sub.2 O.sub.3) is an inorganic color pigment, C.I. Pigment Green 17, having a corundum-hematite crystalline structure. It is commonly used to impart a green color to ceramics, paints, polymers, and other materials. The DCMA Classification and Chemical Description of the Complex Inorganic Color Pigments, Third Addition (1991), published by the Dry Color Manufacturer's Association, states that its composition may include any one or a combination of the modifiers Al.sub.2 O.sub.3 (alumina), Fe.sub.2 O.sub.3 (iron oxide), or Mn.sub.2 O.sub.3 (manganese oxide).
Chromium green-black hematite is one of the principle pigments used in the manufacture of green shade military camouflage paint and netting. In such applications, chromium green-black hematite is combined with cobalt bearing mixed metal oxides, such as cobalt containing spinel pigment V12600 available from Ferro Corporation of Cleveland, Ohio. This combination of pigments is effective in simulating the reflectivity of chlorophyl in the visible portion of the electromagnetic spectrum, being that portion of the spectrum which is viewable by the naked eye with wavelengths ranging from approximately 0.40 .mu.m to 0.70 .mu.m.
Chlorophyl, which is an organic pigment, generally exhibits a relatively uniform high degree of reflectivity in the near infrared, being that portion of the electromagnetic spectrum with wavelengths ranging from approximately 0.7 .mu.m, to 2.5 .mu.m. Cobalt, however, exhibits a strong absorption band (i.e., low reflectivity) in a portion ofthe near infrared with wavelengths ranging from approximately 1.2 .mu.m to 1.6 .mu.m. In recent years, advancements in imaging technology have made it possible to contrast known military green shade camouflage painted or covered objects from the background foliage in that portion of the near infrared. A substitute military green shade camouflage pigment which contains no cobalt and which closely matches the reflectivity of chlorophyl in the visible and near infrared is therefore highly desired.
In order to satisfy military specifications, a substitute green shade camouflage pigment would have to exhibit a dark drab green appearance in the visible portion of the spectrum and would also have to simulate the reflectance curve for chlorophyl in the near infrared. Generally speaking, known inorganic pigments which exhibit a low degree of reflectivity in the visible portion of the light spectrum (i.e., dark drab colored pigments) also tend to exhibit a correspondingly low degree of reflectivity (i.e., high absorption) in other portions of the light spectrum, including the near infrared. A chromium green-black hematite pigment manufactured by Bayer Corporation of Germany, product number AC 5303, was observed to exhibit a higher near infrared reflectance than other chromium green-black hematite sources (this pigment, however, does not have the desired dark drab appearance in the visible spectrum required for military green shade camouflage paint applications). It was found by chemical analysis that this pigment contained both alumina and titania (basic chemical formula: TiO.sub.2) as minor additives. A search failed to disclose any references teaching the use of alumina and titania to improve the near infrared reflectance of chromium green-black hematite pigments.